What of da Vinci as a philosopher of nature? Perhaps the verdict of his contemporaries should count for something. Their reaction to his voluminous writings on science was simply to ignore them, although da Vinci's habit of using mirror-writing didn't help. His manuscripts were rapidly dispersed as collectors sought out the drawings they contained, while disregarding the words. Then, from the late 19th century and especially following the da Vinci-mania launched by the theft of the Mona Lisa in 1911, his philosophy was reassessed. 

Initially, this new look at da Vinci's scientific notebooks caused some excitement. He rapidly gained a reputation as the ultimate Renaissance man, ahead of his time and struggling against the orthodoxies of his day. But this assessment was the result of a lack of awareness about the achievements of medieval science and astronomy. 

For instance, da Vinci's scientific method consisted of careful observation followed by rational analysis that should lead to necessary laws. This is exactly what basic textbooks in Aristotle's logic had taught generations of medieval students at the great universities of Paris, Oxford and Padua. Da Vinci did occasionally talk about the importance of controlled experiments. But he did not practise what he preached. For example, he agreed with Aristotle that heavy objects must fall faster than light ones. Proving that this is false requires only a very simple experiment: drop a brick and pebble from head-height. Da Vinci seems never to have bothered performing it. His notebooks also discuss impetus and motion in a way that seems to anticipate Galileo and Newton. However, historians now know that Galileo inherited many of his ideas from medieval natural philosophers such as John Buridan, who was active in the 1340s. Da Vinci's work turns out to be mainstream and derivative.

At one point in his notebooks da Vinci wrote in large letters, apparently at random, "the sun does not move". But elsewhere, he accepts the view that the sun, moon and other planets orbit the earth. A century earlier, Nicholas of Cusa had proposed in his bestselling On Learned Ignorance that the earth moves in a circle. He had even explained why we cannot feel this motion. Repeating an argument developed by John Buridan, Nicholas compared the earth to a ship. Without a fixed reference point, he said, when we are on a ship on a smooth sea observing another ship, we cannot tell which of them is actually moving. Likewise, when we observe the stars moving across the sky, we cannot tell whether it is the stars or the earth moving. This argument, later used by Copernicus and Galileo, does not appear in da Vinci's work. As for Nicholas of Cusa, the Pope made him a cardinal.